In general, current mirrors are circuits that are designed to “copy” a current driven through a first active device, such as a transistor, by controlling the current in a second active device, such as another transistor. Such circuits generally keep the output current constant regardless of loading. The “copied” current may be a varying signal current. Typical current mirrors may include a current amplifier which boosts the available drive current to an output device. Current mirrors are often used to provide bias currents and active loads to output devices.
FIG. 1 illustrates a first example of a circuit 100 that implements a prior current mirror having an input portion 101 and an output device 160. A current source 105 is electrically coupled with the input portion 101 of the current mirror which creates gate volte for the output device 160, which includes, for example, a first transistor 110 that has a gate 112, a drain 114, and a source 116 that is electrically coupled to ground. In the example, the output device 160 is a second transistor 160 that has a gate 162, a drain 164, and a source 166 that is electrically coupled to ground.
The gates 112 and 162 of the first and second transistors 110 and 160, respectively, are electrically coupled with each other. Either or both of the first and second transistors 110 and 160 may be each transistor may be a metal-oxide-semiconductor, field-effect transistor (MOSFET). The input supply voltage to the circuit 100 is Vdd, the voltage of the input of the current mirror at the drain 114 of the first transistor 110 is Vgs, and the output voltage at the drain 164 of the second transistor 160 is Vload. However, in situations where the input device is a 1 uA diode connected input device and the output device has up to 200 uA, for example, the circuit 100 is too slow for use where there is a need to settle bias currents in a 40 ns clock cycle.
FIG. 2 illustrates a second example of a circuit 200 that implements a prior current mirror that includes an input portion 201 and an output device 260. A current source 205 is electrically coupled with the input portion 201 of the current mirror which creates gate volte for the output 260. The input portion 201 includes three transistors 210, 220, and 230. The first transistor 210 has a gate 212, a drain 214, and a source 216 that is electrically coupled to ground. The second transistor 220 has a gate 222, a drain 224, and a source 226 that is electrically coupled to ground.
The third transistor 230 has a gate 232, a drain 234, and a source 236 that is electrically coupled with the gate 212 of the first transistor 210 as well as the gate 222 and drain 224 of the second transistor 220. The gate 232 of the third transistor 230 is electrically coupled with the drain 214 of the first transistor 210.
The circuit 200 also includes an output device such as, for example, a fourth transistor 260 that has a gate 262, a drain 264, and a source 266 that is electrically coupled to ground. The gates 212 and 262 of the first and fourth transistors 210 and 260, respectively, are electrically coupled with each other. The input voltage to the circuit 200 is Vdd, the voltage of the input of the current mirror at the drain 214 of the first transistor 210 is Vgs of the second transistor 220 plus Vgs of the third transistor 230, and the output voltage at the drain 264 of the fourth transistor 260 is Vload.
This circuitry arrangement is problematic in that there is minimal headroom at the drain 214 of the first transistor 210. In this circuit 200, the third transistor 230 is a source follower, also referred to herein as a current amplifier, and the second transistor 220 is a bias device for the source follower. Inclusion of the amplifier device 230 improves the current drive capability for better settling.
Thus, there remains a need for improved circuit designs that implement a current mirror.